Methods for producing stereographic (3-D) images or photographs are, in general, well-known. Specialized photographic equipment is employed to capture a number of views of a subject from different angles. The respective views are then merged to create a composite (typically lineiform) image. Classicly, a composite image has been generated by photographing the subject from a plurality of angular positions onto a film having a "taking screen" (line or lenticular) disposed in front of the film. The screen is shifted slightly with respect to the film at each angular position so that a series of vertical line images corresponding to the angular position at which the photograph is taken is provided at each screen position, resulting in a lineiform image. Suitable apparatus for generating composite images is described in U.S. Pat. No. 3,666,465, issued to Winnek on May 30, 1972. Another technique for generating composite images includes photographing the subject from a number of positions, onto a film maintained in a common plane at each of the positions. The camera lens is shifted relative to the film image plane in accordance with the position to maintain the same field of view extremities relative to the subject and the same focus point on the subject relative to a line from the center of the lens. Such an apparatus is described in U.S. Pat. No. 4,724,449 issued to Wright on Feb. 9, 1988.
Typically, a composite image is subjected to a three-step process to realize the stereographic effect. The composite image is printed on a substrate (e.g., paper). A clear coating, typically plastic, is then applied over the composite image, and a screen or grating, in accordance with the method employed to generate the composite image (e.g. a lenticular screen for a lineiform image) is embossed in the coating in registry with the image. Reference is made to U.S. Pat. Nos. 3,420,663, 3,462,226, and 2,297,846 issued to Huffaker on Jan. 7, 1969, Huffaker et al. on Aug. 19, 1969, and von Benschoten on Oct. 6, 1942, respectively.
It is recognized that precise registry between the image and the overlying embossed screen must be maintained. For example, the lenticules of an embossed lenticular screen must correspond precisely in number to the number per inch in an underlying lineiform image, and precisely parallel the lines of the image. In the absence of such registry, the stereoscopic effect is inconsistent, at best, due to, for example, generation of moire pattern. With respect to lineiform composite images, a mismatch of as little as 0.005 line per inch can cause unacceptable objectional confusion.
Reproduction of the 3-D pictures has, in the past, been effected employing a sheet fed lithographic printing press. The composite image is printed on individual sheets, which are then taken to a separate laminating machine where the coating is applied and embossed with the screen. Examples of such systems are described in the aforementioned Huffaker U.S. Pat. No. 3,462,226, and von Benschoten U.S. Pat. No. 2,297,846.
Sheet fed apparatus, however, are not capable of operating at speeds comparable to typical operating speeds of a web offset printing press, e.g., 1600 ft./min. Thus, a process that would permit generation of the stereoscopic images at web offset press speeds has long been needed in the industry. However, web offset presses have typically not been employed to reproduce composite images for stereographic pictures.
The present inventor has determined that composite images printed by conventional web offset printing press, are generally not suitable for providing acceptable stereographic pictures, due, primarily, to difficulty in maintaining registration between the composite image and the embossed screen. Web offset presses typically employ inks, which require drying at relatively high temperatures in order to evaporate solvents in the ink. The drying step, however, also tends to cause the paper to shrink, thus distorting the image printed thereon. Such shrinkage tends to be inconsistent, variable, and nonlinear. Shrinkage tends to vary with paper type, from one roll of paper of a given type to the next, and even within a given roll of paper, depending upon, e.g., variations in moisture content of the paper, and the instantaneous tension on the web. Thus, the images on the web after the drying process tend to vary in size and relative position from the image as printed, (e.g., reflected on the printing plates). While such size and positioned deviations are not significant in most applications, the critical registry between the image and image overlying screen in a stereographic print tends to be lost, resulting in an unacceptable stereographic print.